Obesity is a condition that affects hundreds of millions of people. Obesity is associated with several very serious illnesses that can lead to decreased quality of life and in some cases to early death. Although there are multiple treatment options available for obesity, there is still a significant unmet medical need and a need for new therapies.
The World Health Organization (WHO) defines obesity using an index called the Body-Mass Index (BMI) which is defined as the weight of an individual in kilograms divided by the square of the height in meters. Individuals with a BMI greater than 25 kg/m2 are considered overweight, while individuals with a BMI greater than 30 kg/m2 are considered obese. Worldwide, approximately 1.6 billion adults are overweight and 400 million adults are obese, and these numbers are expected to increase [WHO Fact Sheet No. 311, September 2006].
Individuals with high BMIs are at risk of a number of serious complications, and the risk increases with BMI. Among the conditions associated with obesity are cardiovascular disease, diabetes, osteoarthritis, and certain cancers. These conditions cause a reduction in the ability of individuals to lead normal lives, and may lead to early death.
Obesity results from an imbalance between the intake of calories in food (for example, in carbohydrates and fats) and energy expenditure (for example, in exercise). A number of factors including genetic susceptibility have been found to contribute to the likelihood that an individual will become obese.
Stearoyl-CoA desaturase (SCD) is an enzyme which catalyzes the introduction of a cis double bond into saturated fatty acids to give monounsaturated fatty acids. Two of the most important substrates for the SCD enzymes are the Co-enzyme A esters of stearate and palmitate, which are converted into oleate and palmitoleate respectively. Oleate is the most common monounsaturated fatty acid found in membrane phospholipids, triglycerides, and cholesterol esters, and the ratio of saturated to unsaturated fatty acids affects membrane fluidity. The ratio of a monounsaturated fatty acid to the corresponding saturated fatty acid (for example, the ratio between oleate and stearate) is known as the desaturation index.
Several different isoforms of the SCD enzyme are known, and the number and tissue expression of the different isoforms vary across different species. For example, in the mouse, four different isoforms of the enzyme are known (SCD1, SCD2, SCD3, and SCD4), while in human two forms of SCD are known (SCD1 and SCD5). The homology between the human and mouse SCD1 proteins is 85% [L. Zhang et al. Biochem. J. 1999, 340, 255-264] while the two human isoforms of the protein have 75% homology. Human SCD1 is highly expressed in liver and especially in adipose tissue, while SCD5 has the highest levels of expression in brain and pancreas [J. Wang et al. Biochem. Biophys. Res. Commun. 2005, 332, 735-742].
The potential of SCD1 as a target for the treatment of obesity is shown from expression data in humans, from a naturally occurring mutation in mice, from an SCD1 knockout mouse, and by reducing the expression of the SCD1 protein using antisense oligonucleotides.
Transcriptional profiling of RNA from a small sample of lean and obese donors revealed that mRNA expression of SCD1 was elevated three-fold in the obese individuals while other genes involved in the oxidation of fatty acids, such as pyruvate dehydrogenase kinase 4, carnitine palmitoyltransferase 1β, and malonyl-CoA decarboxylase did not differ significantly between the two groups [M. H. Hulver et al. Cell Metab. 2005, 2, 251-261]. Furthermore, the SCD1 mRNA levels showed a positive association with BMI; the desaturation index of total tissue lipids (for oleate/stearate) was 40% higher in muscle lipid extracts from obese donors; and fatty acid oxidation was higher in primary human skeletal myocytes from lean than from obese donors. Studies in animals also show that SCD1 expression is higher in obese than in lean individuals. For example, C57BI/6 mice fed a high-fat diet have SCD1 mRNA levels 50% higher than those of mice on a low fat diet. SCD1 activity was also shown to be 50% higher in liver microsomes from the high-fat fed mice, by measuring the rate of production of oleate from stearate [S. B. Biddinger et al. Diabetes 2005, 54, 1314-1323].
The asebia (abJ/abJ) mouse is a mutant strain of BALB/c mice. The mutation arose spontaneously and results in a lack of functional SCD1 because of the deletion of the first four exons of the gene. The phenotype of the asebia mouse includes alopecia and skin defects which are not seen in the heterozygotes [A. H. Gates and M. Karasek Science 1965, 148, 1471-1473]. In addition, the asebia mice show decreased levels of triglycerides and liver cholesterol esters [M. Miyazaki et al. J. Biol. Chem. 2000, 275, 30132-31038]. From studies in animals in which both the SCD1 and ob genes are defective, a greater understanding of the relevance of SCD1 in the development of obesity has emerged.
The ob/ob mouse is one of the most common models used in obesity research. In this model, the mouse has a mutation in the gene that codes for the 16 kDa hormone leptin, which plays a role in the regulation of appetite and energy expenditure, and the mutation renders the mice obese with enlarged livers engorged with fat. Expression of the SCD1 gene is increased in the liver of leptin-deficient ob/ob mice, compared to wild-type mice, and the overexpression of SCD1 is reduced by administration of leptin [P. Cohen et al. Science 2002, 297, 240-243]. Furthermore, intercrossing ob/ob mice with asebia mice results in double mutant abJ/abJ; ob/ob mice which have increased lean mass compared to the ob/ob mice, but lower body weight and lower fat mass, despite consuming more food. These results suggest that downregulation of SCD1 is one mechanism through which leptin acts, and also that SCD1 is indeed a viable target for pharmacological intervention for the treatment of obesity.
The phenotype of an SCD1 knockout mouse further supports the validity of this enzyme as an obesity target. The SCD1−/− mouse was generated by targeted disruption of the SCD1 gene in C57BI/6 mice. The SCD1−/− mice have reduced body fat and are resistant to weight gain when fed a high-fat diet. They have increased energy expenditure and increased oxygen consumption. Genes involved in lipid oxidation are over-expressed in these mice, while genes involved in lipid synthesis are down-regulated [J. M. Ntambi et al. Proc. Natl. Acad. Sci USA 2002, 99, 11482-11486]. Homozygous SCD1 knockout mice exhibit abnormalities in the sebaceous gland as do the asebia mice described above, and also in the meibomian gland in the eye.
SCD1-specific antisense oligonucleotides (ASOs) have been shown to reduce SCD1 mRNA and protein levels in mouse primary hepatocytes. In addition, fatty acid synthesis in primary hepatocytes is reduced while fatty acid oxidation increases. In C57BI/6 mice fed on a high fat diet for 10 weeks, SCD1-specific ASOs led to a significant reduction in weight gain, without an effect on food intake. The percentage of fat is decreased and the ratio of lean mass to total body mass is increased in the ASO-treated animals. Examination of the livers after 10 weeks showed that ASO-treatment resulted in a reduction in de novo fatty acid synthesis and also in hepatic steatosis [G. Jiang et al. J. Clin. Invest. 2005, 115, 1030-1038]. The alopecia phenotype of the asebia mouse was not observed in the treated animals. Three patent applications (WO 2005014607, US 2004254359, WO 2003012031) describe antisense compounds, compositions and methods for modulating the expression of stearoyl-CoA desaturase. These compounds, compositions and methods are claimed to be useful for the treatment of diseases associated with the expression of SCD, including atherosclerosis, cardiovascular diseases and abnormal cholesterol or lipid metabolism.